How to create an Unchained multisig vault with two Coldcards

Coldcard is a premium hardware wallet for sophisticated users, and if that’s you, then you probably understand why you would want to use multisig to secure your bitcoin and eliminate single points of failure.

Our platform is built to take full advantage of the Coldcard’s multisig features, and we’re pleased to support Coldcard’s air-gapped functionality. In addition to our general guide for setting up an Unchained account yourself, we’ve put together this article to walk you through the nuances of adding your keys, creating your multisig vault, and signing transactions specifically on the Coldcard.

A note about Concierge Onboarding

Before we get started, note that our Concierge team is available through the Concierge Onboarding package to walk you through every step of this process. With our white-glove Concierge Onboarding service you get an onboarding call with a vault specialist, training on how to use our open-source external recovery tools, exclusive access to continuing education webinars, ongoing support, and 90 days of access to our Concierge Client service. With Concierge Client, you can get expert help well beyond the initial setup.

Things you’ll need

Unchained Capital supports Coldcard models Mk2, Mk3 and Mk4. In addition to the device themselves, some additional accessories are required.

If you’re using a Coldcard Mk2 or Mk3, you need the accessories to support moving data between your personal computer and the Coldcard using a MicroSD card. These devices cannot directly interface with your computer via USB.

• Micro-USB cable to connect the device to power
• MicroSD card (32GB or less)
• MicroSD card reader for your computer

The latest Mk4 model of the Coldcard supports both air-gapped functionality and direct connection via USB-C:

• USB-C cable to connect the device to power (or to your computer if you will not be using the device air-gapped)
• Optional for air-gapped functionality: MicroSD card (32GB or less) 
• Optional for air-gapped functionality: MicroSD card reader for your computer

Create an Unchained Capital Account

The first step is creating an Unchained Capital account. Log in at my.unchained.com and fill out the required onboarding information depending on which products you want to use. Tier 2 gives individuals and businesses the ability to use vaults. Tier 3 is required for bitcoin OTC purchases and for USD loans.

Export the xpub from the Coldcard

The first step is exporting the xpub from the Coldcard. If you’re using the Mk2 or Mk3 air-gapped, you’ll need to export this file to your MicroSD card.

1. Unlock your Coldcard with your PIN.
2. Insert your MicroSD card (unless you are using the Mk4 and are connecting it to computer directly).
3. Navigate to Advanced.
4. For Mk2 / Mk3 only: Select MicroSD.
5. Select Export Wallet.
6. Select Unchained Capital.
7. Review the message then hit the checkmark button.
8. If you are using a MicroSD card, move it to your MicroSD card reader on your computer.

Create an Unchained Capital key by uploading your xpub

1. Log into your Unchained Capital Account
2. Navigate to the keys menu and select create a new key
3. Name the key, and write down the key name on your recovery seed
4. Select Coldcard
5. Select Upload xpub and select the exported file from the MicroSD card (Note: You’ll find the file on your computer’s file explorer in the location “Coldcard” if you are using an Mk4 without a MicroSD card) 
6. Your xpub will appear on Unchained Capital if it has been imported correctly. Select next and then create

See the full key creation flow below. Click any image to enlarge.

Create a second key

You can either create a second Coldcard key by performing the above steps with a second initialized Coldcard, or upload a second key by using a Trezor or Ledger and following the onscreen instructions.

Build a vault

Once you have 2 keys uploaded, you can build a vault by selecting “create a new vault” from your dashboard or by selecting New Vault from the menu on the side.

Initialize the multisig vault on your Coldcard

After you build your vault, select “Configure Coldcard” from the Coldcard Users screen or navigate to the menu in the completed Vault’s transact window and download the Coldcard configuration file. This file will teach your Coldcard about the vault and different keys used to secure it.

1. Move the Coldcard configuration file from your downloads folder onto the MicroSD card  (or directly to your connected Mk4)
2. Remove the MicroSD card from the computer and plug it into your Coldcard
3. On your Coldcard homescreen, select Settings, then Multisig Wallets
4. Select Import from SD or Import from File
5. The Coldcard will find the file. Select ok, then select the file
6. Review the multisig wallet and select ok
7. Now your Coldcard can sign partially signed bitcoin transactions (PSBTs) produced by the vault.

Sign a PSBT from the Vault with a Coldcard

1. Author a withdrawal from the vault dashboard and select your Coldcard key as one of the signers
2. In the signing workflow, select the key secured by the Coldcard and select that you are signing from a Coldcard
3. Download the PSBT file and transfer it to the Coldcard with the MicroSD card (or Mk4 with direct connection) 

4. On your Coldcard, select ready to sign. Your Coldcard will find the PSBT. Select it and hit ok
5. Review the transaction details. If they look good, hit ok to sign the transaction. Your Coldcard will produce a new file
6. Transfer the signed PSBT file from your Coldcard to your computer with the MicroSD card (or Mk4 with direct connection).

You have signed the transaction with one key. You can now proceed to request Unchained co-sign a transaction, or sign it with a second key and broadcast it.